1. Field of the Invention
The present invention relates generally to image forming apparatuses such as a copier, a printer, a facsimile machine, and a multifunction peripheral including these functions, and more particularly to an image forming apparatus employing an electrophotographic method, in which a toner image is formed on an image carrier by performing charging, writing, and developing, the toner image is directly transferred or indirectly transferred via an intermediate transfer body onto a recording material, and the transferred toner image is fixed with a fixing device, thereby recording an image onto the recording material such as a sheet. Furthermore, the present invention relates generally to fixing devices included in such image forming apparatuses for fixing unfixed toner onto the recording material after the toner image is transferred, and more particularly to a fixing device for fixing the toner image onto the recording material by applying fixing liquid while the recording material is being conveyed.
2. Description of the Related Art
Presently, there is a wide variety of apparatuses, such as copiers, printers, and facsimile machines, for forming an image on a recording sheet (recording material), such as a sheet, a cloth, or an OHP transparency, according to image information. Among them, the predominant type of apparatus employs the electrophotographic method in which toner is used to form an image on plain paper at high speed and with high density and high precision. Furthermore, in recent years and continuing, these electrophotographic image forming apparatuses can conveniently create color images, and are thus being widely used in offices.
In the image forming apparatus employing the electrophotographic method, a charging device, a writing device, a developing device, a transfer device, a cleaning device, and a charge eliminator are arranged around an image carrier that is drum-shaped or belt-shaped. As the image carrier rotates, a charging operation is performed and then a writing operation is performed so that an electrostatic latent image is formed on the surface of the image carrier. The electrostatic latent image is then developed by causing toner to adhere to the latent image, thus creating a toner image on the image carrier. Then, the toner image is directly transferred or indirectly transferred via a belt-like intermediate transfer body onto a recording material, thereby recording an image on the recording material. After the toner image has been transferred, the transfer material is conveyed to a fixing device, and the unfixed toner is fixed on the recording material. Meanwhile, after the image has been transferred, the surface of the image carrier is cleaned by the cleaning device and static electricity is eliminated from the surface of the image carrier, so that the image carrier is prepared for another image forming operation.
In the electrophotographic method, heat is often employed for the fixing operation, because the fixing speed is fast and the image quality of the fixed image is high. Specifically, a heating element such as a halogen heater or a ceramic heater is used to heat a roller or a film. A recording sheet on which unfixed toner is placed is sandwiched by a pair of fixing rollers (heating rollers and pressurizing rollers) to be heated and pressurized. Accordingly, the toner is dissolved and deformed in such a manner as to be fixed onto the recording sheet by being anchored to the fiber of the recording sheet.
This method is widely used due to its superior uniformity and reliability. However, the problem with this method is excessive power consumption. In recent years and continuing, energy conservation is an important issue. Thus, there are considerable requirements for low-power-consuming fixing devices, instead of those that employ the heat fixing method. Furthermore, fixing devices that employ the heat fixing method require a long time to start up before commencing the fixing operation. Conventionally, there have been proposed techniques for mitigating this problem or to improve various existing fixing methods (see, for example, Patent Documents 1 through 10).
As described in Patent Document 1, there have long been two fixing methods, i.e., a heating method and a steam fixing method. In the steam fixing method, a recording sheet on which unfixed toner is placed is inserted into solvent vapor. This method surely consumes less power than the heating method, but did not become widespread because the liquid has an unpleasant odor and may harm the human body. However, a new type of liquid has been recently developed. This liquid is odorless, harmless, and is capable of causing toner to expand, dissolve, and to be fixed on sheets. Therefore, the fixing method employing liquid is attracting attention once again.
As color copiers have become predominant, multiple layers of toner are often superposed onto a recording sheet, so that the total height of toner becomes 20 μm through 25 μm. Assuming that the entire A4-sized sheet is covered with a solid image containing layers of toner as high as this, there needs to be enough fixing liquid for filling all of the gaps in the toner. Experimental results show that the required amount of fixing liquid is approximately 1 milliliter per A4-sized sheet. Accordingly, in order to fix toner onto 10,000 sheets, 10 liters of fixing liquid are necessary, which would require a huge tank. Furthermore, a curling phenomenon would occur, in which one side of the recording sheet expands and warps because of the liquid. As a result, the quality of printouts is significantly decreased.
Patent Document 2 describes a method for reducing the amount of liquid used. This method attempts to reduce the required amount of liquid by applying the liquid only onto portions where an image is formed, and not applying the liquid onto portions without any images.
Patent Documents 3 through 5 describe a technology for jetting fixing liquid onto a toner image on the recording sheet and then performing a pressurizing step, a technology for charging capsules containing fixing liquid and inducing them to an electrode arranged on a side opposite to a supplying unit, and a technology for turning the fixing liquid into a mist including microscopic liquid droplets of around 0.5 μm through 5 μm.
However, in the method described in Patent Document 2, the fixing liquid is applied according to image position information, and therefore, it is difficult to cause the fixing liquid to adhere at precise positions. As a result, the configuration becomes complex. Furthermore, the fixing liquid is not applied to toner on background stains, which do not corresponding to image position information. Hence, the toner of the background stains will be in an unfixed status on the ejected sheet. As a result, the unfixed toner may soil the user and/or the surrounding environment.
In the method described in Patent Document 4, toner capsules are charged, and a line-type electrode is provided behind the recording sheet. The line-type electrode applies a voltage onto the recording sheet according to image signals in order to pull the toner capsules. This configuration is complex, and selections can only be made in units of lines.
Patent Document 6 describes a fixing device that uses odorless and harmless fixing liquid. Specifically, the fixing liquid includes a softening agent for softening toner and a solvent for dispersing or dissolving the softening agent. This fixing liquid is sprayed or dripped, or applied with the use of a roller, so that the fixing liquid adheres to a recording material onto which an image has been transferred. The fixing liquid softens the toner, and then the fixing liquid is dried so that the unfixed toner is fixed onto the recording material.
The fixing device employing this method does not require a toner heating process as in the case of the heat fixing method. Accordingly, this method consumes low power, and is thus appropriate for energy conservation measures. However, various problems arise due to applying a large amount of fixing liquid onto the recording material.
Specifically, the problems are described in the following. 1) If water is used as the solvent in the fixing liquid, a large amount of water will expand the fiber in the sheet of paper used as the recording material. As a result, the dissolved toner will reach the back side of the recording material, i.e., the image will ooze through to the back side, thus degrading image quality. 2) If a large amount of this water is absorbed, the recording material will be wrinkled or curled. This will have a significantly adverse effect on the operation performed by the image forming apparatus of conveying the recording material in a reliable manner and at high speed. 3) In order to remove a large amount of this water by causing it to evaporate with the use of a drying device, the amount of power consumption will be the same as that used in the heat fixing method. 4) Because a large amount of fixing liquid is used, it is necessary to frequently replace the fixing liquid. If a large-capacity fixing liquid tank is used, the fixing device will need to be large. For this reason, a compact-sized image forming apparatus may not be realized.
In order to solve the above problems 1) through 4), several fixing methods have been conventionally proposed. For example, Patent Documents 7 and 10 describe inkjet methods in which the fixing liquid is applied only to the toner image based on position information of the toner image on the recording material or position information on the toner image on the image carrier before being transferred onto the recording material. However, with this method, the fixing liquid needs to be accurately applied according to the image data in units of the toner, which is difficult to realize.
In a fixing device described in Patent Document 6, if liquid droplets actually strongly strike the toner, the toner will be forced to move, because the toner is adhering to the recording material only by static electricity. As a result, the toner image on the recording material will be distorted. Accordingly, Patent Documents 8 and 9 propose a transfer fixing device. Specifically, the fixing liquid that has been turned into liquid droplets is charged, and then caused to adhere to a secondary transfer roller. The fixing liquid is caused to adhere to toner on the secondary transfer roller by a Coulomb force. Then, the toner is transferred onto a recording material together with the fixing liquid droplets, to be fixed on the recording material. By employing this method, small droplets strike the toner at a relatively low speed, and therefore, the kinetic energy is small and the toner is prevented from being moved.
In such a conventional fixing device, the fixing liquid droplets adhering to the secondary transfer roller are directly transferred together with the toner onto the recording material so that the toner is fixed onto the recording material. However, in order to reliably fix the toner, a considerable amount of fixing liquid droplets needs to be applied to the recording material. As a result, the surface of the recording material will become wet, and the recording material will become curled.
Patent Document 1: Japanese Published Examined Patent Application No. S40-10867
Patent Document 2: Japanese Laid-Open Patent Application No. 2004-109751
Patent Document 3: Japanese Laid-Open Patent Application No. 2004-294847
Patent Document 4: Japanese Laid-Open Patent Application No. 2006-133306
Patent Document 5: Japanese Laid-Open Patent Application No. 2004-333866
Patent Document 6: Japanese Patent No. 3290513
Patent Document 7: Japanese Laid-Open Patent Application No. 2004-109747
Patent Document 8: Japanese Laid-Open Patent Application No. 2006-163083
Patent Document 9: Japanese Laid-Open Patent Application No. 2006-293169
Patent Document 10: Japanese Laid-Open Patent Application No. 2006-350099